1. Field of the Invention
The invention relates to an electromagnetic clutch, for use with a compressor for an automobile air conditioning system, improved to prevent a compressor lubricating oil from entering the contacting surfaces between a rotor and an armature.
2. Description of the Related Art
In an electromagnetic clutch for use with a compressor for an automobile air conditioning system of a prior art, as indicated in FIG. 6A, a compressor oil collected at a ring shield provided on an inner circumference of a rotor of the clutch, or the oil that moves along a rotational shaft of the compressor may spatter to invade contacting surfaces 202 through arcuate slots 201 for providing a magnetic path.
Japanese Examined Utility Model Publication (Kokoku) No.4-3133 discloses a compressor with a back cover attached to an end wall of an armature to prevent the invasion of compressor lubricating oil leaking out through a shaft seal of the compressor into contacting surfaces of the armature and a rotor of an electromagnetic clutch so that the loss of friction-engagement force at the contacting surfaces is reduced. With reference to FIG. 6B, the prior art prevents invasion of oil through arcuate slots 201 because a back cover provided at the end wall of the armature plate closes the arcuate slots in the armature plate.
However, the electromagnetic clutch described above encounters a problem in that the noise caused by the sudden compression of the air between an armature and a rotor that takes place when the electromagnetic coil is energized is accelerated due to the provision of the back cover.
When the armature is attracted to the rotor, the air between the armature and the rotor is suddenly compressed to become compressed air, and is discharged through the gap between the armature and the rotor. The discharged air generates shock wave which reaches the human ear as noise. In the prior art, as shown in FIG. 7, the escape route for the compressed air is limited to the gap between the armature and the rotor only to interfere with smooth discharge of the compressed air. The excessive compression of the air is even increased compared with case without a back cover because the back cover 204 attached to the end wall of the armature plate 203 closes the arcuate slots 201, resulting in a problem of further increased noise caused by the shock wave.